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Noun

An illusion is a distortion of the senses,
revealing how the brain
normally organizes and interprets sensory stimulation. While
illusions distort reality, they are generally shared by most
people. Illusions may occur with more of the human senses than vision, but visual
illusions, optical
illusions, are the most well known and understood. The emphasis
on visual illusions occurs because vision
often dominates the other senses. For example, individuals watching
a ventriloquist
will perceive the voice is coming from the dummy since they are
able to see the dummy mouth the words. Some illusions are based on
general assumptions the brain makes during perception. These assumptions
are made using organizational principles, like Gestalt, an
individual's ability of depth
perception and motion perception, and perceptual
constancy. Other illusions occur because of biological sensory
structures within the human body or
conditions outside of the body within one’s physical
environment.

The term illusion refers to a specific form of
sensory distortion. Unlike a hallucination, which is a
distortion in the absence of a stimulus, an illusion describes a
misinterpretation of a true sensation. For example, hearing voices
regardless of the environment would be a hallucination, whereas
hearing voices in the sound of running water (or other auditory
source) would be an illusion.

Mimes are known for a repertoire of illusions
that are created by physical means. The mime artist
creates an illusion of acting upon or being acted upon by an unseen
object. These illusions exploit the audience's assumptions about
the physical world. Well known examples include "walls", "climbing
stairs", "leaning", "descending ladders", "pulling and pushing"
etc.

Optical illusions

An optical
illusion is always characterized by visually
perceived images that, at least in common sense terms, are
deceptive or misleading. Therefore, the information gathered by the
eye is processed by the brain to give, on the face of it, a
percept that does not
tally with a physical measurement of the stimulus source. A
conventional assumption is that there are physiological illusions
that occur naturally and cognitive illusions that can be
demonstrated by specific visual tricks that say something more
basic about how human perceptual systems work. The human brain
constructs a world inside our head based on what it samples from
the surrounding environment. However sometimes it tries to organise
this information it thinks best while other times it fills in the
gaps. This way in which our brain works is the basis of an
illusion.

Auditory illusions

An auditory
illusion is an illusion of hearing,
the sound equivalent of an
optical illusion: the liner hears either sounds which are not
present in the stimulus, or "impossible" sounds. In short, audio
illusions highlight areas where the human ear and brain, as
organic, makeshift tools, differ from perfect audio receptors (for
better or for worse). One of example of an auditory illusions is a
Shepard
tone.

Tactile illusions

Examples of tactile illusions include phantom
limb, the thermal
grill illusion, the cutaneous
rabbit illusion and a curious illusion that occurs when the
crossed index and middle fingers are run along the bridge of the
nose with one finger on each side, resulting in the perception of
two separate noses. Interestingly, the brain areas activated during
illusory tactile perception are similar to those activated during
actual tactile stimulation. Tactile illusions can also be elicited
through haptic technology. These "illusory" tactile objects can be
used to create "virtual objects"

Other senses

Illusions can occur with the other senses
including that of taste and smell. It was discovered that even if
some portion of the taste receptor on the tongue became damaged
that illusory taste could be produced by tactile stimulation..
Evidence of olfactory
(smell) illusions occurred when positive or negative verbal labels
were given prior to olfactory stimulation.

Disorders

Some illusions occur as result of an illness or a
disorder. While these types of illusions are not shared with
everyone they are typical of each condition. For example migraine
suffers often report Fortification
illusions.